People misplace commonly used objects, or especially forget where they have left them, for example in a home or place of work. If the object is small, such as a set of keys or glasses, it may be difficult to find and the search may take a long time. This is why various wireless transmission location systems have already been proposed. The present invention proposes the use of wireless UVB (Ultra Wide Band) technology in a system of this type. The main advantages of using this technology in short distance location of objects are mentioned in US Patent Application 2006/0033662. However, to date, UWB signals used in these applications had to be combined with narrow band radiofrequency signals.
US Patent Application No. 2008/0136644 discloses in detail a system of searching for and locating objects including identification tags which contain both narrow band RF circuits and UWB circuits. This system notably enables the distance between a local search device and an object sought to be measured using UWB technology, by measuring the return travel time of the signals. This document specifies that UWB technology enables relatively short distances to be measured with sufficient precision to locate objects, typically precision on the order of a decimetre or even a centimeter. However, the proposed system involves quite a complex structure and also requires a synchronisation operation to be performed between the search device and the tags for each search.
Another system using UWB signals for short distance object location is disclosed in EP Patent Application No 1 630 966 and one of the embodiments thereof has the features set out in the above preamble. This system is mainly intended for searching for a person who has a mobile terminal, for example a telephone, which constitutes the object sought and the target device within the meaning of the above description. The search method includes two successive phases. The first comprises responding to an emergency call, for example from the mobile terminal, by generally locating the mobile terminal, for example by means of GPS, and sending a rescue team to the location. The second phase comprises short distance location, generally to find victims, and uses the exchange of UWB signals between the mobile terminal sought and the search device held by the rescue crew, as explained below.
The mobile terminal emits characteristic UWB signals on demand, either by manual pressure on a key, or automatically in response to a location request, sent for example by short text message, or in response to detection of a dangerous situation by the terminal itself. To locate the mobile terminal using UWB signals, the search device includes four antennae arranged at the points of a rhombus and associated in pairs with an electronic circuit that measures the differences in the UWB signal reception time in each pair of antennae. From these measurements, the search device calculates the orthogonal coordinates of the mobile terminal relative to the frame of reference defined by the four antennae, then the polar coordinates (azimuth and distance) and displays the latter by means of a hand and a digital display.
The system disclosed in the aforecited document EP 1 630 966 cannot easily be applied to searching for misplaced objects, since the mobile terminal, which cannot be manually activated in such case, becomes too complicated to take the form of a tag, for example, and to have a standby mode with almost zero power consumption. Another drawback results from the need for four antennae placed at the points of a rhombus, which means the search device must have a relatively large volume.